There are genetically modified chickens, but it is my understanding that these are not currently used in the meat industry. The genetically-modified chickens are used by bio-pharmaceutical companies (the chickens have been modified to produce special proteins that can be harvested from eggs for medical purposes). Others may know more about this, and I defer to them.
The current 'standard' meat chicken is the product of a highly selective breeding program. The result is an oddity that certainly would never survive in nature, but there is nothing (all things being equal) particularly harmful or unhealthy about it. It may not be as flavourful (IMHO), but it is still normal chicken meat.
The downfalls are minimal, such as leg & heart problems. Those I know who raise meat birds kill/process the birds before such problems can manifest themselves. I have found, however, that the CornishX's cannot handle heat/cold nearly as well as other chicken breeds (but keep in mind they were bred to be raised in indoor cage facilities, not the backyard). Most I know try to avoid raising theirs in the heat of mid-Summer.
I will be using the Rangers this Spring. This has less to do with meat, and more with gardening. I planned my garden design around using 4 chicken tractors to go in-between my garden rows. CornishX's are worthless for working the soil and weeding, and the Rangers are more active foragers. I need a meat bird that is willing to work a little before meeting his/her Maker!